Handle an archive with just wupgets

Here's an archive with just wupgets https://github.com/sprunk/just_wupgets

This kind of archive would be useful as e.g. minimal repro example for Lua API, or just so a newbie developer can copy-paste a bunch of wupgets into a game and get something working.

But the wupgets aren't actually loaded. Fix this, or at least document the hurdles.

Start with the linked just_wupgets archive and trace how RecoilEngine loads archives containing only wupgets. Reproduce the missing-load behavior, then determine whether the archive can be loaded or which hurdles must instead be documented; done means the minimal archive works or its limitations are clearly documented.

Written by the indexing model from the issue text.